Nonaqueous electrolytic solution for electric double layer capacitors

A nonaqueous electrolytic solution for electric double layer capacitors prepared by dissolving a quaternary ammonium salt represented by formula (I):

• Q+X− (I),
• wherein Q+ represents a quaternary ammonium cation comprising pyrrolidinium, and X− represents a counter ion selected from the group consisting of PF6−, BF4−, AsF6−, N(CF3SO3)2−, SbF6−, and RfSO3−, wherein Rf is a fluoroalkyl group having 1 to 8 carbon atoms, as an electrolyte in a nonaqueous solvent,
• wherein:
• the quaternary ammonium salt excludes spiro quaternary ammonium tetrafluoroborate,
• the nonaqueous solvent is at least one solvent selected from the group consisting of propylene carbonate, ethylene carbonate, butylene carbonate, sulfolane, methylsulfolane, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, γ-butyrolactone and mixtures thereof, and
• an alkali metal cation concentration is more than 10 ppm and not more than 30 ppm.